The ion implantation of energetic impurity atoms into a silicon substrate to change its electronic properties is commonplace in the electronics industry. The trend in the industry is toward increasing complexity of integrated circuits and decreasing device dimensions. This requires more rigid tolerances and lower defect densities. Shallow doping profiles are often required and, as a consequence thereof, processing temperatures should be lowered from over 1000.degree. C., the range in conventional use, to below 900.degree. C., preferably substantially below 900.degree. C.
Lower processing temperatures are required to minimize diffusion of the implant and consequent loss of resolution, thereby achieving tighter tolerances and smaller dimensions for devices. Lower processing temperatures are also desirable in that they increase the number and variety of materials that can be present in or on the substrate when implantation is carried out. There is, in general, a quest in the semiconductor industry for lower processing temperatures in all phases of device manufacture.
For an ion-implanted impurity to be electrically active, it must be located in substitutional sites in the crystal lattice of the surface layer of the substrate after annealing. When the damage caused to the crystal lattice by ion implantation is not sufficient to amorphize the surface of a silicon substrate, lower processing temperatures may not cause the implant to become substitutional. Therefore, annealing temperatures of at least about 1000.degree. C. are required to make the implant electrically active. Such high temperatures are undesirable in that they cause diffusion from the implant which has an adverse effect on the implant profile.
When there is sufficient damage caused by the ion implant of the impurity to amorphize the surface of the silicon substrate, lower processing temperatures may produce sufficient regrowth of the amorphized silicon by solid-phase epitaxy to restore the surface with minimal crystal defects and have the implanted ions located in substitutional sites. A low-dose implant, i.e. below about 10.sup.15 ions/cm.sup.2, of phosphorus, arsenic or boron will not produce the desired degree of damage in the silicon substrate to contemplate the use of lower processing temperatures. It has been necessary, therefore, to either utilize annealing temperatures of at least about 1000.degree. C. or to create the necessary degree of damage for such implants to become electrically active at lower processing temperatures.
One method of creating the damage necessary for an impurity implant to become substitutional upon annealing is to implant molecular ions, for example, implanting boron as BF.sup.+ or BF.sub.2.sup.+ instead of the pure ion, i.e. B.sup.+. Molecular ions, in sufficient concentration, produce enough damage in the surface of the crystalline silicon substrate to create an amorphous layer, whereas a pure ion implant of B.sup.+ requires a significantly higher annealing temperature to obtain high sheet conductivity in the layer. The advantage of implanting molecular ions can be demonstrated by forming diodes with B.sup.+, BF.sup.+ and BF.sub.2.sup.+ implants at 10.sup.15 ions per square centimeter. The BF.sub.2.sup.+ implanted diodes clearly demonstrate the lowest leakage current. The advantages of using molecular ions such as BF.sup.+,BF.sub.2.sup.+, PF.sup.+, AsF.sup.+ and the like, however, do not hold true for low-dose implants which leave the silicon surface substantially unamorphized.
Another known method of creating the damage necessary for an impurity ion implant to become substitutional is to pre-implant the surface of the silicon to damage or amorphize it. While it is known that any atom can amorphize silicon, such amorphizing ion implants are generally carried out with inert ions such as silicon, neon or argon. Active ions, such as the halogens, are generally avoided for amorphizing implants.
An additional active ion is utilized as an amorphizing implant, however, in the process disclosed in MacIver et al., U.S. Pat. No. 4,144,100, issued Mar. 13, 1979. Implantation of such ions, e.g. fluorine or chlorine, is carried out coextensive with or preferably, following a low-dose phosphorus ion implant. The &lt;100&gt; p-type silicon substrate is then annealed in an oxidizing atmosphere at a temperature of at least 900.degree. C., preferably at 1000.degree.-1200.degree. C. It is stated that amorphizing ion implants with inert gases or nitrogen were not of any significant benefit in the stated object of preventing leakage from the diodes formed by the process.
In accordance with this invention, a method of forming an electrically effective low-dosage ion implant of phosphorus, arsenic or boron has been found which advantageously, does not require high annealing temperatures. That the subject process utilizes lower than conventional annealing temperatures is a significant improvement in view of the ongoing need for lower processing temperatures in semiconductor device manufacturing.